Added tests for line continuity

Added tests for line continuity

The current tests verify line rasterization results
quite loosely because of differences in implementations.
However a line with missing pixels were still passing
the tests. This adds separate tests that catch such
rasterization errors.

Amber has been updated to add polygon mode support.

New tests:

dEQP-VK.rasterization.line_continuity.line-strip
dEQP-VK.rasterization.line_continuity.polygon-mode-lines

VK-GL-CTS issue: 2024

Components: Vulkan
Change-Id: Ibad1deb85f3a5bbc169723f0d8c00c3192447917